1. Field of the Invention
This invention relates to an electrophotographic image forming apparatus, such as a laser printer, an analog copy machine, or a digital copy machine, for forming a monochrome image or a color image having various densities.
2. Description of the Related Art
Conventional image-copying machines have a drawback in which they sometimes produce copies from a single original having different densities. Such variations in image density results from changes or deterioration of the characteristics of image-forming materials and components, which occur as a result of the influence of ambient conditions over time. In the case of a multigradation printer or digital copy machine, as well as an analog copy machine, it is desirable that variations in image density be kept to a minimum. In particular, stable image density is an indispensable factor in obtaining good color images, since this influences not only density reproductivity but also color reproductivity. Accordingly, stabilization of image density has been achieved by manually performing a maintenance operation by an operator within an allowance imparted to the characteristics of image-forming materials and components, and also to the image-forming process.
However, imparting an allowance to image-forming materials, components, or the process increases the cost of the apparatus, and the manual maintenance causes the increase of the cost and labor. Further, since the image density varies in the course of a cycle which is shorter than the usual maintenance cycle, a stable image density cannot be maintained over a long time period merely by maintenance work.